This invention relates to measuring blood pressure.
Systems for measuring blood pressure are generally known. During a measurement cycle, a blood pressure cuff secured around the patient's limb is inflated to a sufficiently high pressure to cut off arterial blood flow beneath the cuff, and the cuff is incrementally deflated to allow the artery to slowly open. As the cuff is deflated, biological signals indicative of blood pressure (such as sounds, known as Korotkoff sounds, caused by the blood forcing its way through the artery) are detected by a transducer on the cuff and converted to electrical signals that are processed to determine the systolic and diastolic blood pressures. This type of measurement technique is known as auscultation.
Other blood pressure measurement techniques are known. In oscillometry, small pressure changes in an inflated cuff induced by flowing blood are detected by a transducer (disposed either on the cuff or at a remote monitor) and used as a basis for determining blood pressure. Another procedure involves using multiple transducers to detect the times of occurrence of heart pulses at different locations along the artery, and determining the blood pressure based on the pulse propagation time between the transducers.
Often it is clinically useful to measure blood pressure (by any of the techniques described above, or possibly by other techniques) during critical care periods (for example, while the patient is undergoing surgery or being treating in an intensive care unit), or as the patient exercises to, for example, monitor how blood pressure changes with variations in heart rate. But activity, by the patient or by others, during these times generates noise (i.e., signals that are not indicative of blood pressure) that may be incorrectly interpreted, resulting in inaccurate blood pressure measurement.
In some systems that measure blood pressure by auscultation, a threshold based on the level of previously received, valid Korotkoff sounds is applied to all signals produced by the transducer as a discriminant to remove noise. Some systems use two transducers that are spaced on the cuff to provide a half-period delay between the Korotkoff sounds and take the difference between the signals produced by the sensors to reinforce the Korotkoff sounds; because noise appears generally the same at each transducer, the noise level in the difference signal is reduced.
U.S. Pat. No. 5,392,781, entitled "Blood Pressure Monitoring in Noisy Environments" and assigned to the present assignee (the "'781 patent"), describes several techniques for aiding in the discrimination of blood pressure signals from noise.